A new study found consumers were more likely to donate blood when tempted by offers like store gift cards and other freebies than those who signed up without any perks.

Shocker.

Analysts from The National Bureau of Economic Research studied the donation rates of nearly 100,000 individuals at 72 American Red Cross sites in Northern Ohio.

Half of the centers advertised offers for $5, $10, and $15 gift cards, which prompted short-term spikes in donations, especially among those who were older and had donated in the past.

Five-dollar rewards led to 3.5% more donations in repeat visitors while $10 and $15 rewards drew 6.9% and 9.5% more, respectively.

"We find that individuals who were offered economic rewards to donate blood were not only more likely to donate, and more so the higher the value of the rewards, but were also more likely to attract others to donate," the study says.

But as soon as the "carrot" was taken away, donations began to dive, leading analysts to conclude that rewards only increased giving in the short-term.

Surprise rewards turned out to be less of a draw for donations, as consumers showed they weren't willing to get pricked without knowing what was in it for them upfront.

Those who were given a surprise reward after donating were even less likely to repeat donations than those who were given no goodies, the study found.

Overall, people were less likely to donate with the promise of a "surprise" than those who donated without any reward promise at all. Researchers say this could be because the idea of receiving some type of gift for donating blood might blur consumers' motivation for giving in the first place.